1. Field of the Invention
This invention is concerned with micro-electro-mechanical systems (MEMS) and in particular with high-performance MEMS devices with comb actuators.
2. Description of the Related Art
Micro-electro-mechanical systems (MEMS) fabricated from single crystal silicon by anisotropic etching are widely used for sensors (accelerometers, for example) and are of increasing commercial interest and importance for use in a variety of active devices, such as electrical switches, variable capacitors and inductors, and micromirrors for optical scanning and switching. A typical MEMS active device comprises a functional element that is anchored by a spring or hinge but suspended above a substrate so that it can be moved by an actuator, such as a capacitively driven comb structure. The moveable functional element might be a switch contact, capacitor plate or micromirror, for example. In addition to the small size desirable for portable equipment, MEMS devices offer the potential for faster response times, lower power consumption and reduced costs. Large cost benefits can be provided if the yield of functional devices per processed wafer is high.
An important potential application for MEMS is scanning mirrors, which are used in a wide variety of measurement and communications equipment including barcode readers, laser printers, confocal microscopes and fiber-optic networks. Compared to macro-scale scanning mirrors, MEMS micromirrors offer faster scanning speed, lower power consumption and reduced cost, and are enabling with respect to many new technologies. In particular, scanning micromirrors with high frequency optical switching capability are critical to development of advanced telecommunications systems.
The state of the art for design of active MEMS devices is illustrated by the scanning micromirror device with a comb actuator described in a recent publication (R. A. Conant, J. T. Lee, N. Y. Lau and R. S. Muller, p. 6, Proc. Solid-State and Actuator Workshop, Hilton Head Island, S.C., Jun. 4-8, 2000). For this device, one silicon layer comprises a coplanar circular mirror (550 xcexcm diameter) and a moveable comb actuator suspended via a silicon torsion spring, which is connected to a stationary anchor. A second silicon device layer comprises a stationary comb structure whose teeth are immediately below the spaces between the teeth in the moveable comb. Capacitive charging of the teeth on the two combs by an applied voltage produces a force of attraction that tends to move the moveable comb and the attached micromirror, which are returned to their original positions by the torsion spring when the voltage is removed. In this prior art micromirror design, mechanical support for the device is provided by the device layer containing the stationary comb structure so that this layer must be relatively thick. Also, the two device layers are bonded together via a silicon oxide layer, which must be partially removed in one of the final steps of fabrication to release the moveable structure. This silicon oxide layer between the two device layers is formed during a fusion bonding process that operates at high temperature (1100xc2x0 C.).
This prior art design, which is typical of prior art MEMS devices, has several significant disadvantages. For example, a hole must be etched through the thick support device layer to provide access to the micromirror, which greatly increases the processing time (3-5 hours), reduces the etching accuracy, and requires use of thicker photoresist with reduced feature resolution. Furthermore, the wet chemical etch required for release, typically hydrofluoric acid (HF) or buffered oxide etch (BOE), is difficult to control and greatly reduces the yield of useable devices. In addition to requiring a high processing temperature, the silicon-oxide-silicon bonding process is very sensitive to particulates so that the wafers must be handled in at least a Class 10 cleanroom environment. Also, the presence of deep etched features on both sides of the device complicates photolithographic processing and increases the likelihood of damage during handling. Likewise, use of the stationary comb silicon layer as one of the electrical connections limits options for device integration and in some cases may necessitate extra processing to electrically ground or connect floating elements. These disadvantages of prior art MEMS devices reduce the precision of alignment between the stationary and moveable comb structures, which further reduces the device yield and necessitates use of excessively high actuator voltages. Although typical of the prior art MEMS fabrication processes, all of these disadvantages may not apply equally to every device and every fabrication process.
The present invention provides a MEMS device with two silicon device layers. The first silicon device layer comprises a stationary actuator comb structure (with an electrical contact area) and bond pads, which are disposed on an insulating layer (preferably, silicon oxide) on a substantially continuous substrate layer (preferably, silicon). The second device layer comprises a moveable functional device element and a moveable actuator comb structure connected via at least one silicon spring to stationary silicon bond pads. Mating bond pads on the two silicon device layers are bonded together (preferably by thermal compression bonding) via a metal layer, which is preferably comprised of an adhesion layer (chromium, titanium, nickel or cobalt, for example) and a gold layer. A voltage applied between the actuator structures on the two silicon layers tends to cause the moveable comb structure and functional device element to move relative to the stationary bond pads, and the spring tends to restore them to their original positions in the absence of applied voltage. Voltages are applied via electrical contacts to the actuator structures, which preferably comprise metallic contact pads and wire bonds. Devices according to the present invention have device features on only one side of the device.
The device of the present invention offers significant advantages compared to prior art MEMS devices. Use of a supporting substrate enables the silicon device layers to be relatively thin, which greatly reduces silicon etch times. In addition to reducing costs, short silicon etch times improve the accuracy and yield of the etch processes and permit use of high resolution photoresists providing higher feature resolution. Use of metal-metal interlayer bonds avoids the need for a wet chemical etch for release of the moveable structure so that the device can be fabricated with high yield. Metal-metal thermal compression bonding is also less sensitive to particulates, compared to the silicon-silicon oxide bonding of the prior art, and can be performed in a relatively low-quality cleanroom environment (Class 1000, for example). The device design of the present invention also enables precise alignment between the silicon device layers, and use of low-temperature metal-metal bonding minimizes thermal distortion of the device elements.
Scanning micromirror devices according to the present invention were fabricated with almost 100% yield and exhibited scanning over a 12xc2x0 optical range and a mechanical angle of xc2x13xc2x0 at a high resonant frequency of 2.5 kHz with an operating voltage of only 20 VDC. Comparable prior art MEMS scanning micromirror devices fabricated using a prior art process required 100 V for switching and were obtained in only about 20% yield. The present invention encompasses a wide variety of MEMS devices, including but not limited to electrical switches, variable capacitors and inductors, high frequency resonators, and micromirrors for optical scanning and switching. Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.